The vulnerability, identified as CVE-2013-2423, was one of the 42 security issues fixed in Java 7 Update 21 that was released by Oracle last week, on April 16.

According to Oracle's advisory at the time, the vulnerability only affects client, not server, deployments of Java. The company gave the flaw's impact a 4.3 out of 10 rating using the Common Vulnerability Scoring System (CVSS) and added that "this vulnerability can be exploited only through untrusted Java Web Start applications and untrusted Java applets."

However, it seems that the low CVSS score didn't stop cybercriminals from targeting the vulnerability. An exploit for CVE-2013-2423 was integrated into a high-end Web attack toolkit known as Cool Exploit Kit and is used to install a piece of malware called Reveton, an independent malware researcher known online as Kafeine said Tuesday in a blog post.

Reveton is part of a class of malicious applications called ransomware that are used to extort money from victims. In particular, Reveton locks down the operating system on infected computers and asks victims to pay a fictitious fine for allegedly downloading and storing illegal files.

Security researchers from Finnish antivirus vendor F-Secure confirmed the active exploitation of CVE-2013-2423. The attacks started on April 21 and were still active as of Tuesday, they said in a blog post.

The vulnerability started being targeted by attackers one day after an exploit for the same flaw was added to the Metasploit framework, an open-source tool commonly used by penetration testers, the F-Secure researchers said.

This wouldn't be the first time when cybercriminals have taken Metasploit exploit modules and adapted them for use with their own malicious attack toolkits.

Users who need Java on their computers and especially in their browsers are advised to upgrade their Java installations to the latest available version -- Java 7 Update 21 -- as soon as possible. This version also made changes to the security warnings displayed when websites attempt to load Web-based Java applications in order to better represent the risk associated with allowing different types of applets to execute.

Users should only agree to run Java applets from websites that they trust and which normally load such content. Browsers like Google Chrome and Mozilla Firefox also have a feature known as click-to-play that can be used to block plug-in-based content from executing without explicit consent.